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STEP-NC Traceability Pilot Project

Learn about the OPC standards for CNC machines, data logging with virtual machines, and methods for efficient data analysis and reporting.

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STEP-NC Traceability Pilot Project

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  1. STEP-NC Traceability Pilot Project Fred Proctor, Mike McGlauflin and John Michaloski NIST Manufacturing Engineering Laboratory

  2. Today, OLE for Process Control (OPC) has emerged as a leading worldwide specification in enabling connectivity and interoperability of factory floor equipment.

  3. CNC OPC Tag Naming & Mapping • OPC does not define any application item tag names • OMAC HMI Working Group has defined a list of CNC-related OPC tags • Object orientation is attained by a tag naming strategy – using slash or period – Axis.Label, Axis.Position • Comparison of Commercial CNC OPC tag collections has been done at NIST against OMAC tags • There are some gaps which we hope to identify

  4. Data Logging Virtual Machine • The predominant CNC machine tool programming language is RS274 • Most CNCs have their own programming language extension to RS274, known as User Macros or Subprograms • Subprograms and Macros provide a layer of programming abstraction on top of the CNC, acting as a Virtual Machine • A Data Logging virtual machine will be specified as a set of subprogram calls that are implemented in the native CNC RS274 dialect

  5. Data Logging Process • CNC with an OPC server provides OPC tagged data • Data Logging OPC client is a separate application running in the background of the CNC computer or a remote computer • You specify a Data list that you wish to collect from the CNC • When one or more values change in the data list, the CNC OPC server will notify the OPC Client of the changes and the OPC Client can read these values.

  6. Data Logging Process • A Data Handler component manipulates the data according to a given configuration • For a more concise data, configure the options of the data collection by defining a Filter. The default filter is to log all the data items whenever one changes. • The Formatter provides support for saving the log to a Destination, such as Persistent Data Store, in some common data collection and analysis format. Typical formats include None, Text, Comma Separated Values (CSV), XML, or database. • A Data Analysis can generate reports, fine tune or troubleshoot machining performance, analyze trends, or tie-in directly to the CNC and provide safeguarding machining operations – such as machine halting or feed rate override.

  7. Method 1: Explicit Naming • AddDataItem(NAME , FILTERTYPE, value1 {, value2}) is implemented in the CNC Virtual Machine as a subprogram or macro • NAME is one of • Axis • AxisLoad, SpindlePower • CmdFeedrate, ActualFeedrate • CmdSpindleSpeed, ActualSpindleSpeed • State, Mode, ProgramName, Block, LineNumber • FILTERTYPE is one of • Deadband: logs only if changes between values is greater than value1 • Threshold: logs only if abs(value) > value1 • Range: logs if value is in [value1 , value2] range • Dependent: unmonitored, logs only if other values change • Equality: supervisory, logs only if value equals value1 • Inequality: supervisory, logs only if value not equal to value1 

  8. Method 1: Explicit Naming • StartLogging(TYPE, FILENAME) • TYPE defines the type of archival destination for the captured data, either NONE, TXT or CSV (text files), or DB (data base) • FILENAME defines the archival destination file name or other appropriate identifier • Problem here is enumerating list of all potential data items. Possible solutions: use categories of data logging, such as axis position, axis dynamics, axis servo, program

  9. Method 2: Delegation • OpenLogging(TYPE, FILE, DATA, FILTER) references files that contain the explicit data • TYPE is one of NONE, TXT, CSV or DB • FILE is the destination file or identifier name • DATA is the file specifying the data to be captured • FILTER is the file specifying the filter to be applied • Example: • OpenLogging(“CSV”, “PartA”, “DataItems”, “”) • Associated Start,Stop,CloseLogging() macros

  10. OPC-CNC Synchronization • Method 2 used for this pilot: • We built an OPC client in C++ that listens to CNC and reads out tagged data • Siemens 840D NC code calls • MSG(OpenLogging(…)) to set up the OPC client • MSG(StartLogging) tells OPC client to begin • NC code continues with machining, while OPC client logs the data according the configuration files

  11. Spindle power log plot, showing spikes in some corners

  12. Spindle power log plot, subset with clean data

  13. Axis path deviation and spindle load

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